This invention relates to a method of using a copper etchant for the removal of copper from substrates such as printed circuit boards. In the production of such boards, conductive circuits are developed by forming a mask over a copper laminate using an etch resistant material such as plastic. The laminate is then exposed to a chemical which etches away the unprotected copper. A typical etchant is composed of hydrogen peroxide and sulfuric acid.
A substantial number of prior art patents have been issued covering various peroxide/sulfuric acid etchants containing numerous additives intended to promote or accentuate certain properties of the etching solution. For example, various organic corrosion inhibitors such as saturated aliphatic acids or esters, sulfonates or sulfonic acids are covered in U.S. Pat. No. 3,412,032. Catalysts such as urea or thiourea (U.S. Pat. No. 3,668,131) or thio sulfate (U.S. Pat. No. 4,130,455) or low molecular weight carboxylic acids (U.S. Pat. No. 4,462,861) are not used to promote etching rates. A number of compounds are mentioned as stabilizers including substituted aniline, sulfones and sulfolanes (U.S. Pat. No. 3,801,512), and oxy quinoline (U.S. Pat. No. 4,022,703).
U.S. Pat. No. 4,141,850 recommends the use of a glycol as a promoter to enhance the dissolution rate of the etching solution in the presence of chloride or bromide ions. However, it discourages the use of either ethylene glycol or propylene glycol for this purpose. U.S. Pat. No. 3,773,577 suggests the addition of ethylene glycol in an amount of 0.5% by weight per volume of the etching solution but with no explanation of its purpose in the solution.
U.S. Pat. No. 4,437,931 describes the use of an acetylenic diol as a promoter in a peroxide etching bath containing free Cl.sup.- or Br.sup.- ions. Examples of suitable polymers include 2 butyne-1, 4-diol, 3-hexyne-2, 5 diol and others.
The copper that is removed from the printed circuit board forms copper sulfate which remains dissolved in the etch bath. As the concentration of the copper sulfate in the etch bath increases, its presence tends to retard the etching rate of the bath. To restore the efficiency of the bath, the copper sulfate is removed and the sulfuric acid and peroxide are replenished. A simple method or removing the copper sulfate is to chill the bath thereby reducing the solubility of the copper sulfate causing it to crystallize and precipitate out as a solid. One way of chilling the bath is to transfer the bath from the treatment tank to an auxiliary tank where refrigeration equipment is used to cool the solution from its operating temperature of 75.degree. F.(24.degree. C.)-140.degree. F.(60.degree. C.) to 32.degree. F.(0.degree. C.)-50.degree. F.(10.degree. C.). At 60.degree. C. the solubility of copper sulfate is 40 gms per 100 ml of water whereas at 10.degree. C., the solubility is about 17.4 gm/100 ml of water, and at 0.degree. C., the solubility of CuSO.sub.4 is about 1/3 of its solubility at 60.degree. C. A problem arises, however, during the chilling of the solution because the copper sulfate as it crystallizes tends to deposit on the sides of the tank and the crystal growth proceeds uninhibited until a hard layer of copper sulfate crystals is firmly deposited on the equipment. Often, it must be removed from the equipment with chisels, pneumatic hammers or the like. Furthermore, the loose crystals that form on the bottom of the tank become too large to be removed by filtering. The unrestrained crystal growth is often enhanced by the presence of the stabilizers in the etch bath.